This invention relates to semiconductor pressure responsive devices. More particularly, it relates to a semiconductor pressure transducer having an extremely thin vapor deposited polycrystalline silicon diaphragm, and to a method of making it.
Semiconductor pressure transducers are currently gaining wide acceptance in a variety of commercial pressure sensing applications. Semiconductor pressure transducers apply the principle that the electrical resistance of resistor in a thin semiconductor diaphragm changes as the function of diaphragm stress due to pressure differentials on either side of the diaphragm. The sensitivity of the devices depend generally on such factors as the thickness of the diaphragm and the piezo-resistive gauge factor of the material used for the diaphragm. Commercially available prior art semiconductor pressure transducers have used single crystal or monocrystalline silicon for the diaphragm. That is, the diaphragm consists of a single, unitary crystal having no grain or crystal boundaries throughout the body. Until now, it was generally accepted that monocrystalline silicon was the most practical diaphragm material which could be used in order to achieve the sensitivity required for most applications.
It has been recognized that for optimum linear response of the pressure transducer, the pressure responsive resistor should be electrically insulated or isolated from the diaphragm and supporting structure. However, the highest commercially available resistivity for monocrystalline silicon is of the order of about 10.sup.2 ohm-centimeter. Therefore, such monocrystalline silicon cannot be readily used for electrical isolation of the pressure responsive resistor. Such isolation has been generally provided by diffusing the monocrystalline silicon with impurities of one conductivity type and by forming the pressure responsive resistor by diffusing impurities of an opposite conductivity type into the diaphragm to provide a PN junction therebetween.
However, certain inherent problems exist with such PN junction isolation. For example, if the PN junction is exposed to ambient conditions, the junction can become contaminated. To alleviate this condition a passivating oxide coating is ordinarily applied over the exposed PN junction. However, this introduces an additional difficulty because the passivating oxide has a different coefficient of thermal expansion than the monocrystalline silicon diaphragm, which imparts undesirable temperature dependence on the response of the device. That is, the output of the device may vary with a change in temperature although there is no change in the pressure being sensed.
In making these monocrystalline silicon diaphragms, one would remove selected portions of a monocrystalline silicon wafer by etching, grinding, lapping or polishing to define the desired thickness for the various diaphragms to be produced from the wafer. However, using these processes one could not readily form diaphragms having a uniform thickness of less than 10 microns. Therefore, the sensitivity of these monocrystalline silicon diaphragms is limited by this parameter. Furthermore, the thickness of the diaphragm is not accurately controllable by these manufacturing techniques. Therefore, consistent reproducibility of extremely thin diaphragms in high volume production is not obtainable using these prior art methods.